Distal antenna and distal antenna related encode nuclear proteins containing pipsqueak motifs involved in antenna development in Drosophila

Novel Antennapedia and Ultrabithorax trimeric complexes with TBP and Exd regulate transcription

BACKGROUND

Hox proteins interact with DNA and many other proteins, co-factors, transcriptional factors, chromatin remodeling components, non-coding RNAs and even the extracellular matrix that assembles the Hox complexes. The number of interacting partners continues to grow with diverse components and more transcriptional factors than initially thought. Hox complexes present many activities, but their molecular mechanisms to modulate their target genes remain unsolved.

RESULTS

In this paper we showed the protein-protein interaction of Antp with Ubx through the homeodomain using BiFC in Drosophila. Analysis of Antp-deletional mutants showed that AntpHD helixes 1 and 2 are required for the interaction with Ubx. Also, we found a novel interaction of Ubx with TBP, in which the PolyQ domain of TBP is required for the interaction. Moreover, we also detected the formation of two new trimeric complexes of Antp with Ubx, TBP and Exd using BiFC-FRET; these proteins, however, do not form a trimeric interaction with BIP2 or TFIIEβ. The novel trimeric complexes reduced Antp transcriptional activity, indicating that they could confer specificity for repression.

CONCLUSIONS

Our results increase the number of transcriptional factors in the Antp and Ubx interactomes that form two novel trimeric complexes with TBP and Exd. We also report a new Ubx interaction with TBP. These novel interactions provide important clues of the dynamics of Hox-interacting complexes involved in transcriptional regulation, contributing to better understand Hox function.

Dan forms condensates in neuroblasts and regulates nuclear architecture and progenitor competence in vivo

Genome organization is thought to underlie cell type specific gene expression, yet how it is regulated in progenitors to produce cellular diversity is unknown. In Drosophila, a developmentally-timed genome reorganization in neural progenitors terminates competence to produce early-born neurons. These events require downregulation of Distal antenna (Dan), part of the conserved pipsqueak DNA-binding superfamily. Here we find that Dan forms liquid-like condensates with high protein mobility, and whose size and subnuclear distribution are balanced with its DNA-binding. Further, we identify a LARKS domain, a structural motif associated with condensate-forming proteins. Deleting just 13 amino acids from LARKS abrogates Dan's ability to retain the early-born neural fate gene, hunchback, in the neuroblast nuclear interior and maintain competence in vivo. Conversely, domain-swapping with LARKS from known phase-separating proteins rescues Dan's effects on competence. Together, we provide in vivo evidence for condensate formation and the regulation of progenitor nuclear architecture underlying neuronal diversification.

Polycomb safeguards imaginal disc specification through control of the Vestigial-Scalloped complex

A fundamental goal of developmental biology is to understand how cell and tissue fates are specified. The imaginal discs of Drosophila are excellent model systems for addressing this paradigm as their fate can be redirected when discs regenerate after injury or when key selector genes are misregulated. Here, we show that when Polycomb expression is reduced, the wing selector gene vestigial is ectopically activated. This leads to the inappropriate formation of the Vestigial-Scalloped complex, which forces the eye to transform into a wing. We further demonstrate that disrupting this complex does not simply block wing formation or restore eye development. Instead, immunohistochemistry and high-throughput genomic analysis show that the eye-antennal disc unexpectedly undergoes hyperplastic growth with multiple domains being organized into other imaginal discs and tissues. These findings provide insight into the complex developmental landscape that tissues must navigate before adopting their final fate.

Pipsqueak family genes dan/danr antagonize nuclear Pros to prevent neural stem cell aging in Drosophila larval brains

Neural stem cell aging is a fundamental question in neurogenesis. Premature nuclear Pros is considered as an indicator of early neural stem cell aging in Drosophila. The underlying mechanism of how neural stem cells prevent premature nuclear Pros remains largely unknown. Here we identified that two pipsqueak family genes, distal antenna (dan) and distal antenna-related (danr), promote the proliferation of neural stem cells (also called neuroblasts, NBs) in third instar larval brains. In the absence of Dan and Danr (dan/danr), the NBs produce fewer daughter cells with smaller lineage sizes. The larval brain NBs in dan/danr clones show premature accumulation of nuclear Prospero (Pros), which usually appears in the terminating NBs at early pupal stage. The premature nuclear Pros leads to NBs cell cycle defects and NB identities loss. Removal of Pros from dan/danr MARCM clones prevents lineage size shrinkage and rescues the loss of NB markers. We propose that the timing of nuclear Pros is after the downregulation of dan/danr in the wt terminating NBs. dan/danr and nuclear Pros are mutually exclusive in NBs. In addition, dan/danr are also required for the late temporal regulator, Grainyhead (Grh), in third instar larval brains. Our study uncovers the novel function of dan/danr in NBs cell fate maintenance. dan/danr antagonize nuclear Pros to prevent NBs aging in Drosophila larval brains.

Subunits of the PBAP Chromatin Remodeler Are Capable of Mediating Enhancer-Driven Transcription in Drosophila

The chromatin remodeler SWI/SNF is an important participant in gene activation, functioning predominantly by opening the chromatin structure on promoters and enhancers. Here, we describe its novel mode of action in which SWI/SNF factors mediate the targeted action of an enhancer. We studied the functions of two signature subunits of PBAP subfamily, BAP170 and SAYP, in Drosophila. These subunits were stably tethered to a transgene reporter carrying the hsp70 core promoter. The tethered subunits mediate transcription of the reporter in a pattern that is generated by enhancers close to the insertion site in multiple loci throughout the genome. Both tethered SAYP and BAP170 recruit the whole PBAP complex to the reporter promoter. However, we found that BAP170-dependent transcription is more resistant to the depletion of other PBAP subunits, suggesting that BAP170 may play a more critical role in establishing enhancer-dependent transcription.

In vivo analysis of the evolutionary conserved BTD-box domain of Sp1 and Btd during Drosophila development

The Sp family of transcription factors plays important functions during development and disease. An evolutionary conserved role for some Sp family members is the control of limb development. The family is characterized by the presence of three C2H2-type zinc fingers and an adjacent 10 aa region with an unknown function called the Buttonhead (BTD) box. The presence of this BTD-box in all Sp family members identified from arthropods to vertebrates, suggests that it plays an essential role during development. However, despite its conservation, the in vivo function of the BTD-box has never been studied. In this work, we have generated specific BTD-box deletion alleles for the Drosophila Sp family members Sp1 and buttonhead (btd) using gene editing tools and analyzed its role during development. Unexpectedly, btd and Sp1 mutant alleles that lack the BTD-box are viable and have almost normal appendages. However, in a sensitized background the requirement of this domain to fully regulate some of Sp1 and Btd target genes is revealed. Furthermore, we have also identified a novel Sp1 role promoting leg vs antenna identity through the repression of spineless (ss) expression in the leg, a function that also depends on the Sp1 BTD-box.

Identification of genomic enhancers through spatial integration of single-cell transcriptomics and epigenomics

Single-cell technologies allow measuring chromatin accessibility and gene expression in each cell, but jointly utilizing both layers to map bona fide gene regulatory networks and enhancers remains challenging. Here, we generate independent single-cell RNA-seq and single-cell ATAC-seq atlases of the Drosophila eye-antennal disc and spatially integrate the data into a virtual latent space that mimics the organization of the 2D tissue using ScoMAP (Single-Cell Omics Mapping into spatial Axes using Pseudotime ordering). To validate spatially predicted enhancers, we use a large collection of enhancer-reporter lines and identify ~ 85% of enhancers in which chromatin accessibility and enhancer activity are coupled. Next, we infer enhancer-to-gene relationships in the virtual space, finding that genes are mostly regulated by multiple, often redundant, enhancers. Exploiting cell type-specific enhancers, we deconvolute cell type-specific effects of bulk-derived chromatin accessibility QTLs. Finally, we discover that Prospero drives neuronal differentiation through the binding of a GGG motif. In summary, we provide a comprehensive spatial characterization of gene regulation in a 2D tissue.

Epithelial domains and the primordial antennal nervous system of the embryonic grasshopper Schistocerca gregaria

The antenna is a key sensory organ in insects. Factors which pattern its epithelium and the spacing of sensillae will play an important role in shaping its contribution to adaptive behavior. The antenna of the grasshopper S. gregaria has three major articulations: scape, pedicel, and flagellum. During postembryonic development, the flagellum lengthens as segments (so-called meristal annuli) are added at each molt. However, the five most apical annuli do not subdivide; thus, their epithelial domains must already be defined during embryogenesis. We investigated epithelial compartmentalization and its relationship to the developing primordial nervous system of the antenna by simultaneous immunolabeling against the epithelial cell surface molecule Lachesin, against neuron-specific horseradish peroxidase, and against the mitosis marker phospho-histone 3. We found that Lachesin is initially expressed in a highly ordered pattern of "rings" and a "sock" in the apical antennal epithelium of the early embryo. These expression domains appear in a stereotypic order and prefigure later articulations. Proliferative cells segregate into these developing domains and pioneer- and sensory-cell precursors were molecularly identified. Our study allows pioneer neurons, guidepost cells, and the earliest sensory cell clusters of the primordial nervous system to be allocated to their respective epithelial domain. As the apical-most five domains remain stable through subsequent development, lengthening of the flagellum must originate from more basal regions and is likely to be under the control of factors homologous to those which regulate boundary and joint formation in the antenna of Drosophila.

Single cell RNA-sequencing identifies a metabolic aspect of apoptosis in Rbf mutant

The function of Retinoblastoma tumor suppressor (pRB) is greatly influenced by the cellular context, therefore the consequences of pRB inactivation are cell-type-specific. Here we employ single cell RNA-sequencing (scRNA-seq) to profile the impact of an Rbf mutation during Drosophila eye development. First, we build a catalogue of 11,500 wild type eye disc cells containing major known cell types. We find a transcriptional switch occurring in differentiating photoreceptors at the time of axonogenesis. Next, we map a cell landscape of Rbf mutant and identify a mutant-specific cell population that shows intracellular acidification due to increase in glycolytic activity. Genetic experiments demonstrate that such metabolic changes, restricted to this unique Rbf mutant population, sensitize cells to apoptosis and define the pattern of cell death in Rbf mutant eye disc. Thus, these results illustrate how scRNA-seq can be applied to dissect mutant phenotypes.

The function of the Retinoblastoma (Rb) protein is regulated by its cellular environment. Here, the authors perform single cell RNA-sequencing during Drosophila eye development and identify the impact of an Rbf mutation, which sensitises specific cells to apoptosis by changing metabolism.

Two types of sensory proliferation patterns underlie the formation of spatially tuned olfactory receptive fields in the cockroach Periplaneta americana

In the cockroach Periplaneta americana, to represent pheromone source in the receptive space, axon terminals of sex pheromone-receptive olfactory sensory neurons (pSNs) are topographically organized within the primary center, the macroglomerulus, according to the peripheral locations of sex pheromone-receptive single walled (sw)-B sensilla. In this study, we sought to determine when and where pSNs emerge in the nymphal antenna. We revealed two different pSN proliferation patterns that underlie the formation of topographic organization in the macroglomerulus. In nymphal antennae, which lack sw-B sensilla, pSNs are identified in the shorter sensilla, termed sw-A sensilla. Because new sw-A sensilla emerge on the proximal antenna at every molt, topographic organization in the macroglomerulus must be formed by adding axon terminals of newly emerged pSNs to the lateral region in the macroglomerulus at each molt. At the final molt, a huge number of new sw-B sensilla appeared throughout the whole antenna. Sw-B sensilla in the proximal part of the adult antenna were newly formed during the last instar stage, whereas those located in the distal antenna were transformed from sw-A sensilla. This transformation was accompanied by an increase in the number of pSNs. Axon terminals of newborn pSNs in new sw-B sensilla were recruited to the lateral part of the macroglomerulus, whereas those of newborn pSNs in transformed sw-B sensilla were recruited to the macroglomerulus according to the sensillar location. These mechanisms enable an increase in sensitivity to sex pheromone in adulthood while retaining the topographic map formed during the postembryonic development.

Transcription factor TFIIEβ interacts with two exposed positions in helix 2 of the Antennapedia homeodomain to control homeotic function in Drosophila

Homeoproteins contain the conserved homeodomain (HD) and have an important role determining embryo body plan during development. HDs increase their DNA-binding specificity by interacting with additional cofactors outlining a Hox interactome with a multiplicity of protein-protein interactions. In Drosophila, the first link of functional contact with a general transcription factor (GTF) was found between Antennapedia (Antp) and BIP2 (TFIID complex). Hox proteins also interact with other components of Pol II machinery such as the subunit Med19 from Mediator (MED) complex, TFIIEβ and transcription-pausing factor M1BP. All these interactions clearly demonstrate Hox-driven transcriptional regulation, but the precise molecular mechanism remains unclear. In this paper, we focused on the Antp-TFIIEβ protein-protein interface to establish the specific contacts as well as its functional role. Using Bimolecular Fluorescence Complementation (BiFC) in cell culture and in vivo we found that TFIIEβ interacts with Antp through the HD independently of the YPWM motif and the direct physical interaction is at helix 2, specifically aminoacidic positions I32 and H36 of Antp. We also found, through ectopic assays, that these two positions in helix 2 are crucial for Antp homeotic function in head involution, and thoracic and antenna-to tarsus transformations. Interestingly, overexpression of Antp and TFIIEβ in the antennal disc showed that this interaction is required for the antenna-to-tarsus transformation. In conclusion, interaction of Antp with TFIIEβ is important for the functional specificity of Antennapedia, and amino acids 32 and 36 in Antp HD helix 2 are key for this interaction. Our results open the possibility to more broadly analyze Antp-TFIIEβ interaction on the transcriptional control for the activation and/or repression of target genes in the Hox interactome during Drosophila development.

Specification and Patterning of Drosophila Appendages

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophilamelanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.

Cell fate changes induced by a Distal-less enhancer-trap transgene in the Drosophila antennal imaginal disc

The imaginal discs of the genetically tractable model organism Drosophila melanogaster have been used to study cell-fate specification and plasticity, including homeotic changes and regeneration-induced transdetermination. The identity of the reprogramming mechanisms that induce plasticity has been of great interest in the field. Here we identify a change from antennal fate to eye fate induced by a Distal-less-GAL4 (DllGAL4) P-element insertion that is a mutant allele of Dll and expresses GAL4 in the antennal imaginal disc. While this fate change is not induced by tissue damage, it appears to be a hybrid of transdetermination and homeosis as the GAL4 expression causes upregulation of Wingless, and the Dll mutation is required for the fate change. Neither GAL4 expression nor a Dll mutation on its own is able to induce antenna-to-eye fate changes. This plasticity appears to be unique to the DllGAL4 line, possibly due to cellular stress induced by the high GAL4 expression combined with the severity of the Dll mutation. Thus, we propose that even in the absence of tissue damage, other forms of cellular stress caused by high GAL4 expression can induce determined cell fates to change, and selector gene mutations can sensitize the tissue to these transformations.

Drosophila dany is essential for transcriptional control and nuclear architecture in spermatocytes

The terminal differentiation of adult stem cell progeny depends on transcriptional control. A dramatic change in gene expression programs accompanies the transition from proliferating spermatogonia to postmitotic spermatocytes, which prepare for meiosis and subsequent spermiogenesis. More than a thousand spermatocyte-specific genes are transcriptionally activated in early Drosophila spermatocytes. Here we describe the identification and initial characterization of dany, a gene required in spermatocytes for the large-scale change in gene expression. Similar to tMAC and tTAFs, the known major activators of spermatocyte-specific genes, dany has a recent evolutionary origin, but it functions independently. Like dan and danr, its primordial relatives with functions in somatic tissues, dany encodes a nuclear Psq domain protein. Dany associates preferentially with euchromatic genome regions. In dany mutant spermatocytes, activation of spermatocyte-specific genes and silencing of non-spermatocyte-specific genes are severely compromised and the chromatin no longer associates intimately with the nuclear envelope. Therefore, as suggested recently for Dan/Danr, we propose that Dany is essential for the coordination of change in cell type-specific expression programs and large-scale spatial chromatin reorganization.

Transcriptional profiling of olfactory system development identifies distal antenna as a regulator of subset of neuronal fates

Drosophila uses 50 different olfactory receptor neuron (ORN) classes that are clustered within distinct sensilla subtypes to decipher their chemical environment. Each sensilla subtype houses 1-4 ORN identities that arise through asymmetric divisions of a single sensory organ precursor (SOP). Despite a number of mutational studies investigating the regulation of ORN development, a majority of the transcriptional programs that lead to the different ORN classes in the developing olfactory system are unknown. Here we use transcriptional profiling across the time series of antennal development to identify novel transcriptional programs governing the differentiation of ORNs. We surveyed four critical developmental stages of the olfactory system: 3rd instar larval (prepatterning), 8 hours after puparium formation (APF, SOP selection), 40 hrs APF (neurogenesis), and adult antennae. We focused on the expression profiles of olfactory receptor genes and transcription factors-the two main classes of genes that regulate the sensory identity of ORNs. We identify distinct clusters of genes that have overlapping temporal expression profiles suggesting they have a key role during olfactory system development. We show that the expression of the transcription factor distal antenna (dan) is highly similar to other prepatterning factors and is required for the expression of a subset of ORs.

Distal-less homeobox genes of insects and spiders: genomic organization, function, regulation and evolution

The Distal-less (Dll) genes are homeodomain transcription factors that are present in most Metazoa and in representatives of all investigated arthropod groups. In Drosophila, the best studied insect, Dll plays an essential role in forming the proximodistal axis of the legs, antennae and analia, and in specifying antennal identity. The initiation of Dll expression in clusters of cells in mid-lateral regions of the Drosophila embryo represents the earliest genetic marker of limbs. Dll genes are involved in the development of the peripheral nervous system and sensitive organs, and they also function as master regulators of black pigmentation in some insect lineages. Here we analyze the complete genomes of six insects, the nematode Caenorhabditis elegans and Homo sapiens, as well as multiple Dll sequences available in databases in order to examine the structure and protein features of these genes. We also review the function, expression, regulation and evolution of arthropod Dll genes with emphasis on insects and spiders.

The evolutionary conserved transcription factor Sp1 controls appendage growth through Notch signaling

The appendages of arthropods and vertebrates are not homologous structures, although the underlying genetic mechanisms that pattern them are highly conserved. Members of the Sp family of transcription factors are expressed in the developing limbs and their function is required for limb growth in both insects and chordates. Despite the fundamental and conserved role that these transcription factors play during appendage development, their target genes and the mechanisms in which they participate to control limb growth are mostly unknown. We analyzed here the individual contributions of two Drosophila Sp members, buttonhead (btd) and Sp1, during leg development. We show that Sp1 plays a more prominent role controlling leg growth than btd. We identified a regulatory function of Sp1 in Notch signaling, and performed a genome wide transcriptome analysis to identify other potential Sp1 target genes contributing to leg growth. Our data suggest a mechanism by which the Sp factors control appendage growth through the Notch signaling.

Metamorphic labral axis patterning in the beetle Tribolium castaneum requires multiple upstream, but few downstream, genes in the appendage patterning network

The arthropod labrum is an anterior appendage-like structure that forms the dorsal side of the preoral cavity. Conflicting interpretations of fossil, nervous system, and developmental data have led to a proliferation of scenarios for labral evolution. The best supported hypothesis is that the labrum is a novel structure that shares development with appendages as a result of co-option. Here, we use RNA interference in the red flour beetle Tribolium castaneum to compare metamorphic patterning of the labrum to previously published data on ventral appendage patterning. As expected under the co-option hypothesis, depletion of several genes resulted in similar defects in the labrum and ventral appendages. These include proximal deletions and proximal-to-distal transformations resulting from depletion of the leg gap genes homothorax and extradenticle, large-scale deletions resulting from depletion of the leg gap gene Distal-less, and smaller distal deletions resulting from knockdown of the EGF ligand Keren. However, depletion of dachshund and many of the genes that function downstream of the leg gap genes in the ventral appendages had either subtle or no effects on labral axis patterning. This pattern of partial similarity suggests that upstream genes act through different downstream targets in the labrum. We also discovered that many appendage axis patterning genes have roles in patterning the epipharyngeal sensillum array, suggesting that they have become integrated into a novel regulatory network. These genes include Notch, Delta, and decapentaplegic, and the transcription factors abrupt, bric à brac, homothorax, extradenticle and the paralogs apterous a and apterous b.

The BTB-zinc finger transcription factor abrupt acts as an epithelial oncogene in Drosophila melanogaster through maintaining a progenitor-like cell state

The capacity of tumour cells to maintain continual overgrowth potential has been linked to the commandeering of normal self-renewal pathways. Using an epithelial cancer model in Drosophila melanogaster, we carried out an overexpression screen for oncogenes capable of cooperating with the loss of the epithelial apico-basal cell polarity regulator, scribbled (scrib), and identified the cell fate regulator, Abrupt, a BTB-zinc finger protein. Abrupt overexpression alone is insufficient to transform cells, but in cooperation with scrib loss of function, Abrupt promotes the formation of massive tumours in the eye/antennal disc. The steroid hormone receptor coactivator, Taiman (a homologue of SRC3/AIB1), is known to associate with Abrupt, and Taiman overexpression also drives tumour formation in cooperation with the loss of Scrib. Expression arrays and ChIP-Seq indicates that Abrupt overexpression represses a large number of genes, including steroid hormone-response genes and multiple cell fate regulators, thereby maintaining cells within an epithelial progenitor-like state. The progenitor-like state is characterised by the failure to express the conserved Eyes absent/Dachshund regulatory complex in the eye disc, and in the antennal disc by the failure to express cell fate regulators that define the temporal elaboration of the appendage along the proximo-distal axis downstream of Distalless. Loss of scrib promotes cooperation with Abrupt through impaired Hippo signalling, which is required and sufficient for cooperative overgrowth with Abrupt, and JNK (Jun kinase) signalling, which is required for tumour cell migration/invasion but not overgrowth. These results thus identify a novel cooperating oncogene, identify mammalian family members of which are also known oncogenes, and demonstrate that epithelial tumours in Drosophila can be characterised by the maintenance of a progenitor-like state.

Cancer is a multigenic process, involving cooperative interactions between oncogenes or tumour suppressors. In this study, in a genetic screen in the vinegar fly, Drosophila melanogaster, for genes that cooperate with a mutation in the cell polarity (shape) regulator, scribbled (scrib), we identify a novel cooperative oncogene, abrupt. Expression of abrupt in scrib mutant tissue in the developing eye/antennal epithelium results in overgrown invasive tumours. abrupt encodes a BTB-zinc finger transcription factor, which has homology to several cancer-causing proteins in humans, such as BCL6. Analysis of the Abrupt targets and misexpressed genes in abrupt expressing-tissue and abrupt-expressing scrib mutant tumours, revealed cell fate regulators as a major class of targets. Thus, our results reveal that deregulation of multiple cell fate factors by Abrupt expression in the context of polarity disruption is associated with a progenitor-like cell state and the formation of overgrown invasive tumours. Our findings suggest that defective polarity may also be a critical factor in BTB-zinc finger-driven human cancers, and warrants further investigation into this issue.

Developmental coupling of larval and adult stages in a complex life cycle: insights from limb regeneration in the flour beetle, Tribolium castaneum

Background

A complex life cycle, such as complete metamorphosis, is a key innovation that can promote diversification of species. The evolution of a morphologically distinct larval stage is thought to have enabled insects to occupy broader ecological niches and become the most diverse metazoan taxon, yet the extent to which larval and adult morphologies can evolve independently remains unknown. Perturbation of larval limb regeneration allows us to generate larval legs and antennae with altered limb morphologies, which may be used to explore the developmental continuity that might exist between larval and adult appendages. In this study, we determined the roles of several appendage patterning transcription factors, abrupt (ab), dachshund (dac), Distal-less (Dll), and spineless (ss), in the red flour beetle, Tribolium castaneum, during larval appendage regeneration. The functions of these genes in regenerating and non-regenerating limbs were compared using RNA interference.

Results

During limb regeneration, dac and ss were necessary to re-pattern the same larval structures as those patterned during embryogenesis. Removal of these two genes led to larval appendage patterning defects that were carried over to the adult legs. Surprisingly, even though maternal knockdown of ab had minimal effects on limb allocation and patterning in the embryo, it was necessary for blastema growth, an earlier phase of regeneration. Finally, knockdown of Dll prevented the blastema-like bumps from re-differentiating into appendages.

Conclusions

Our results suggest that, similar to vertebrates, the re-patterning phase of Tribolium larval limb regeneration relies on the same genes that are used during embryonic limb patterning. Thus, the re-patterning phase of regeneration is likely to be regulated by taxon-specific patterning mechanisms. Furthermore, Ab and Dll appear to play important roles during blastema proliferation and re-differentiation, respectively. Finally, our results show that continuity exists between larval and adult limb patterning, and that larval and adult leg morphologies may be developmentally coupled. Thus, the evolution of imaginal discs may have been a key step towards completely removing any developmental constraints that existed between larval and adult phenotypes.

Developmentally regulated subnuclear genome reorganization restricts neural progenitor competence in Drosophila

Stem and/or progenitor cells often generate distinct cell types in a stereotyped birth order and over time lose competence to specify earlier-born fates by unknown mechanisms. In Drosophila, the Hunchback transcription factor acts in neural progenitors (neuroblasts) to specify early-born neurons, in part by indirectly inducing the neuronal transcription of its target genes, including the hunchback gene. We used in vivo immuno-DNA FISH and found that the hunchback gene moves to the neuroblast nuclear periphery, a repressive subnuclear compartment, precisely when competence to specify early-born fate is lost and several hours and cell divisions after termination of its transcription. hunchback movement to the lamina correlated with downregulation of the neuroblast nuclear protein, Distal antenna (Dan). Either prolonging Dan expression or disrupting lamina interfered with hunchback repositioning and extended neuroblast competence. We propose that neuroblasts undergo a developmentally regulated subnuclear genome reorganization to permanently silence Hunchback target genes that results in loss of progenitor competence.

Analysis of the transcriptomes downstream of Eyeless and the Hedgehog, Decapentaplegic and Notch signaling pathways in Drosophila melanogaster

Tissue-specific transcription factors are thought to cooperate with signaling pathways to promote patterned tissue specification, in part by co-regulating transcription. The Drosophila melanogaster Pax6 homolog Eyeless forms a complex, incompletely understood regulatory network with the Hedgehog, Decapentaplegic and Notch signaling pathways to control eye-specific gene expression. We report a combinatorial approach, including mRNAseq and microarray analyses, to identify targets co-regulated by Eyeless and Hedgehog, Decapentaplegic or Notch. Multiple analyses suggest that the transcriptomes resulting from co-misexpression of Eyeless+signaling factors provide a more complete picture of eye development compared to previous efforts involving Eyeless alone: (1) Principal components analysis and two-way hierarchical clustering revealed that the Eyeless+signaling factor transcriptomes are closer to the eye control transcriptome than when Eyeless is misexpressed alone; (2) more genes are upregulated at least three-fold in response to Eyeless+signaling factors compared to Eyeless alone; (3) based on gene ontology analysis, the genes upregulated in response to Eyeless+signaling factors had a greater diversity of functions compared to Eyeless alone. Through a secondary screen that utilized RNA interference, we show that the predicted gene CG4721 has a role in eye development. CG4721 encodes a neprilysin family metalloprotease that is highly up-regulated in response to Eyeless+Notch, confirming the validity of our approach. Given the similarity between D. melanogaster and vertebrate eye development, the large number of novel genes identified as potential targets of Ey+signaling factors will provide novel insights to our understanding of eye development in D. melanogaster and humans.

Comparative genomic analysis of Drosophila melanogaster and vector mosquito developmental genes

Genome sequencing projects have presented the opportunity for analysis of developmental genes in three vector mosquito species: Aedes aegypti, Culex quinquefasciatus, and Anopheles gambiae. A comparative genomic analysis of developmental genes in Drosophila melanogaster and these three important vectors of human disease was performed in this investigation. While the study was comprehensive, special emphasis centered on genes that 1) are components of developmental signaling pathways, 2) regulate fundamental developmental processes, 3) are critical for the development of tissues of vector importance, 4) function in developmental processes known to have diverged within insects, and 5) encode microRNAs (miRNAs) that regulate developmental transcripts in Drosophila. While most fruit fly developmental genes are conserved in the three vector mosquito species, several genes known to be critical for Drosophila development were not identified in one or more mosquito genomes. In other cases, mosquito lineage-specific gene gains with respect to D. melanogaster were noted. Sequence analyses also revealed that numerous repetitive sequences are a common structural feature of Drosophila and mosquito developmental genes. Finally, analysis of predicted miRNA binding sites in fruit fly and mosquito developmental genes suggests that the repertoire of developmental genes targeted by miRNAs is species-specific. The results of this study provide insight into the evolution of developmental genes and processes in dipterans and other arthropods, serve as a resource for those pursuing analysis of mosquito development, and will promote the design and refinement of functional analysis experiments.

Functional synthetic Antennapedia genes and the dual roles of YPWM motif and linker size in transcriptional activation and repression

Segmental identity along the anteroposterior axis of bilateral animals is specified by Hox genes. These genes encode transcription factors, harboring the conserved homeodomain and, generally, a YPWM motif, which binds Hox cofactors and increases Hox transcriptional specificity in vivo. Here we derive synthetic Drosophila Antennapedia genes, consisting only of the YPWM motif and homeodomain, and investigate their functional role throughout development. Synthetic peptides and full-length Antennapedia proteins cause head-to-thorax transformations in the embryo, as well as antenna-to-tarsus and eye-to-wing transformations in the adult, thus converting the entire head to a mesothorax. This conversion is achieved by repression of genes required for head and antennal development and ectopic activation of genes promoting thoracic and tarsal fates, respectively. Synthetic Antennapedia peptides bind DNA specifically and interact with Extradenticle and Bric-à-brac interacting protein 2 cofactors in vitro and ex vivo. Substitution of the YPWM motif by alanines abolishes Antennapedia homeotic function, whereas substitution of YPWM by the WRPW repressor motif, which binds the transcriptional corepressor Groucho, allows all proteins to act as repressors only. Finally, naturally occurring variations in the size of the linker between the homeodomain and YPWM motif enhance Antennapedia repressive or activating efficiency, emphasizing the importance of linker size, rather than sequence, for specificity. Our results clearly show that synthetic Antennapedia genes are functional in vivo and therefore provide powerful tools for synthetic biology. Moreover, the YPWM motif is necessary--whereas the entire N terminus of the protein is dispensable--for Antennapedia homeotic function, indicating its dual role in transcriptional activation and repression by recruiting either coactivators or corepressors.

The pipsqueak-domain proteins Distal antenna and Distal antenna-related restrict Hunchback neuroblast expression and early-born neuronal identity

A fundamental question in brain development is how precursor cells generate a diverse group of neural progeny in an ordered manner. Drosophila neuroblasts sequentially express the transcription factors Hunchback (Hb), Krüppel (Kr), Pdm1/Pdm2 (Pdm) and Castor (Cas). Hb is necessary and sufficient to specify early-born temporal identity and, thus, Hb downregulation is essential for specification of later-born progeny. Here, we show that distal antenna (dan) and distal antenna-related (danr), encoding pipsqueak motif DNA-binding domain protein family members, are detected in all neuroblasts during the Hb-to-Cas expression window. Dan and Danr are required for timely downregulation of Hb in neuroblasts and for limiting the number of early-born neurons. Dan and Danr function independently of Seven-up (Svp), an orphan nuclear receptor known to repress Hb expression in neuroblasts, because Dan, Danr and Svp do not regulate each other and dan danr svp triple mutants have increased early-born neurons compared with either dan danr or svp mutants. Interestingly, misexpression of Hb can induce Dan and Svp expression in neuroblasts, suggesting that Hb might activate a negative feedback loop to limit its own expression. We conclude that Dan/Danr and Svp act in parallel pathways to limit Hb expression and allow neuroblasts to transition from making early-born neurons to late-born neurons at the proper time.

Drosophila CtBP regulates proliferation and differentiation of eye precursors and complexes with Eyeless, Dachshund, Dan, and Danr during eye and antennal development

Specification factors regulate cell fate in part by interacting with transcriptional co-regulators like CtBP to regulate gene expression. Here, we demonstrate that CtBP forms a complex or complexes with the Drosophila melanogaster Pax6 homolog Eyeless (Ey), and with Distal antenna (Dan), Distal antenna related (Danr), and Dachshund to promote eye and antennal specification. Phenotypic analysis together with molecular data indicate that CtBP interacts with Ey to prevent overproliferation of eye precursors. In contrast, CtBP,dan,danr triple mutant adult eyes have significantly fewer ommatidia than CtBP single or dan,danr double mutants, suggesting that the CtBP/Dan/Danr complex functions to recruit ommatidia from the eye precursor pool. Furthermore, CtBP single and to a greater extent CtBP,dan,danr triple mutants affect the establishment and maintenance of the R8 precursor, which is the founding ommatidial cell. Thus, CtBP interacts with different eye specification factors to regulate gene expression appropriate for proliferative vs. differentiative stages of eye development.

Retinal determination the beginning of eye development

The road to producing an eye begins with the decision to commit a population of cells to adopting an eye tissue fate, the process of retinal determination. Over the past decade and a half, a network of transcription factors has been found to mediate this process in all seeing animals. This retinal determination network is known to regulate not only tissue fate but also cell proliferation, pattern formation, compartment boundary establishment, and even retinal cell specification. The compound eye of the fruit fly, Drosophila melanogaster, has proven to be an excellent experimental system to study the mechanisms by which this network regulates organogenesis and tissue patterning. In fact the founding members of most of the gene families that make up this network were first isolated in Drosophila based on loss-of-function phenotypes that affect the eye. This chapter will highlight the history of discovery of the retinal determination network and will draw attention to the molecular and biochemical mechanisms that underlie our understanding of how the fate of the retina is determined.

Function and specificity of synthetic Hox transcription factors in vivo

Homeotic (Hox) genes encode transcription factors that confer segmental identity along the anteroposterior axis of the embryo. However the molecular mechanisms underlying Hox-mediated transcription and the differential requirements for specificity in the regulation of the vast number of Hox-target genes remain ill-defined. Here we show that synthetic Sex combs reduced (Scr) genes that encode the Scr C terminus containing the homedomain (HD) and YPWM motif (Scr-HD) are functional in vivo. Synthetic Scr-HD peptides can induce ectopic salivary glands in the embryo and homeotic transformations in the adult fly, act as transcriptional activators and repressors during development, and participate in protein-protein interactions. Their transformation capacity was found to be enhanced over their full-length counterpart and mutations known to transform the full-length protein into constitutively active or inactive variants behaved accordingly in the synthetic peptides. Our results show that synthetic Scr-HD genes are sufficient for homeotic function in Drosophila and suggest that the N terminus of Scr has a role in transcriptional potency, rather than specificity. We also demonstrate that synthetic peptides behave largely in a predictable way, by exhibiting Scr-specific phenotypes throughout development, which makes them an important tool for synthetic biology.

Mutual regulation of the Drosophila disconnected (disco) and Distal-less (Dll) genes contributes to proximal-distal patterning of antenna and leg

The Drosophila disconnected (disco) gene encodes a C(2)H(2)-type zinc finger transcription factor required for the development of the central and peripheral nervous systems. We report that disco participates in a positive feedback loop with the Dll gene, a master regulator of ventral appendage development. Dll function is not only required for proper disco expression in antenna and leg discs, but is also sufficient for ectopic expression of disco in the developing retina and wing imaginal discs. Conversely, disco gene function is required for the maintenance of Dll expression. We show that Dll phenotypes are partially rescued by the up-regulation of disco expression in the Dll domain. Reduction in disco gene function disrupts antenna and leg development, and the phenotypes closely resemble that produced by Dll alleles. These observations demonstrate that disco plays a fundamental role in the Dll-dependent patterning of antenna and leg, perhaps as a regulator of Dll gene expression.

Cut mutant Drosophila auditory organs differentiate abnormally and degenerate

The Drosophila antenna is a sophisticated structure that functions in both olfaction and audition. Previous studies have identified Homothorax, Extradenticle, and Distal-less, three homeodomain transcription factors, as required for specification of antennal identity. Antennal expression of cut is activated by Homothorax and Extradenticle, and repressed by Distal-less. cut encodes the Drosophila homolog of human CAAT-displacement protein, a cell cycle-regulated homeodomain transcription factor. Cut is required for normal development of external mechanosensory structures and Malphigian tubules (kidney analogs). The role of cut in the Drosophila auditory organ, Johnston's organ, has not been characterized. We have employed the FLP/FRT system to generate cut null clones in developing Johnston's organ. In cut mutants, the scolopidial subunits that constitute Johnston's organ differentiate abnormally and subsequently degenerate. Electrophysiological experiments confirm that adult Drosophila with cut null antennae are deaf. We find that cut acts in parallel to atonal, spalt-major, and spalt-related, which encode other transcription factors required for Johnston's organ differentiation. We speculate that Cut functions in conjunction with these factors to regulate transcription of as yet unidentified targets.

The Tribolium spineless ortholog specifies both larval and adult antennal identity

The morphology of insect antennae varies widely among species, but our understanding of antennal development comes almost solely from studies of one species-the fruit fly, Drosophila melanogaster. Moreover, this knowledge applies mostly to adult structures, since Drosophila lacks external larval appendages. In contrast to Drosophila, the red flour beetle, Tribolium castaneum, has both larval and adult antennae, which are very different from one another in morphology. Thus, Tribolium provides an ideal system to compare modes of antennal development both within and between species. Here, we report that the Tribolium ortholog of spineless (Tc-ss) is required in both the larval and adult antennae. Knockdown of Tc-ss by RNAi during either larval or imaginal development causes transformation of the distal portion of the antennae to legs. Thus, the function of ss is conserved between Drosophila and Tribolium with respect to adult antennal specification and also between Tribolium larval and adult antennal development. The similarity of the Tc-ss RNAi phenotype to that of a classically described Tribolium mutation, antennapedia (ap) (of no relationship to the Drosophila Hox gene of the same name), led us to characterize the original ap mutation and two newly identified ap alleles. Our mapping and phenotypic data suggest that Tc-ss is the best candidate for the ap locus. These results represent a first step in characterizing larval and adult antennal patterning in Tribolium, which should provide important insights into the evolution of insect antennal development.

An antennal-specific role for bowl in repressing supernumerary appendage development in Drosophila

In Drosophila, antennae and legs are serially homologous appendages, and yet they develop into organs of very different structure and function. This implies that different genetic mechanisms operate onto a common developmental ground state to produce antennae and legs. Still few such mechanisms have been uncovered. During leg development, bowl, a member of the odd-skipped gene family, has been shown to participate in the formation of the leg segmental joints. Here we report that, in the antennal disc, bowl has a dramatically different role: bowl is expressed in the ventral antennal disc to prevent inappropriate expression of wg early during development. The removal of bowl function leads to the activation of wg in the dpp-expressing domain. This ectopic expression of wg, together with dpp, results in a new proximo-distal axis that promotes non-autonomous antennal duplications. The role of bowl in suppressing a supernumerary PD axis is maintained even when the antennal disc is homeotically transformed into a leg-like appendage. Therefore, bowl is part of a genetic program that suppresses the formation of supernumerary appendages specifically in the fly's head.

Timing of Wingless signalling distinguishes maxillary and antennal identities in Drosophila melanogaster

The Drosophila adult head mostly derives from the composite eye-antenna imaginal disc. The antennal disc gives rise to two adult olfactory organs: the antennae and maxillary palps. Here, we have analysed the regional specification of the maxillary palp within the antennal disc. We found that a maxillary field, defined by expression of the Hox gene Deformed, is established at about the same time as the eye and antennal fields during the L2 larval stage. The genetic program leading to maxillary regionalisation and identity is very similar to the antennal one, but is distinguished primarily by delayed prepupal expression of the ventral morphogen Wingless (Wg). We find that precociously expressing Wg in the larval maxillary field suffices to transform it towards antennal identity, whereas overexpressing Wg later in prepupae does not. These results thus indicate that temporal regulation of Wg is decisive to distinguishing maxillary and antennal organs. Wg normally acts upstream of the antennal selector spineless (ss) in maxillary development. However, mis-expression of Ss can prematurely activate wg via a positive-feedback loop leading to a maxillary-to-antenna transformation. We characterised: (1) the action of Wg through ssselector function in distinguishing maxillary from antenna; and (2) its direct contribution to identity choice.

Chromosomal binding sites of the homeotic cofactor Homothorax

The Meis family oncoproteins play a crucial role in leukemogenesis and are highly expressed in other types of cancer as well. The transforming potential of Meis proteins depends on their ability to activate gene expression and therefore, revealing the identity of their target genes is very important. The genome of the fruit fly Drosophila melanogaster contains a single Meis gene, homothorax (hth), which plays multiple roles in embryonic and adult development. Mutations in hth affect the development of numerous embryonic and adult tissues, suggesting that Hth regulates the transcription of a large number of genes. However, it is not known how many genes are regulated directly by Hth and what is the nature of these genes. To address this question, we examined the distribution of the in vivo binding sites of Hth on polytene chromosomes. We found that in the salivary glands (SG) of third instar larvae, Hth binds to approximately 150 chromosomal sites in a very reproducible pattern. More than hundred of these sites were mapped cytologically. Interestingly, Hth accumulates at high levels in some of the most prominent hormone-induced chromosomal puffs, pointing to a possible role of Hth in activation of ecdysone-induced targets. Interfering with the normal transcriptional activity of Hth in larval SGs leads to dramatic reduction in cell size and DNA content implicating Hth in the regulation of cell growth and endoreplication in larval SGs.

Additional sex combs affects antennal development by means of spatially restricted repression of Antp and wg

Additional sex combs (Asx) is thought to function in protein complexes of both the Trithorax and Polycomb groups, but very little is known about its developmental roles. Here, we present a detailed analysis of Asx's role in antennal development. We show that loss of Asx in the antennal disc causes a complex phenotype, which consists of distal antenna-to-leg transformations and outgrowth of ectopic leg-like appendages from the Dpp-expressing domain of the disc. Our analyses suggest that these phenotypes are caused mainly by segment-specific de-repression of Antp and expansion of wg expression. We thus conclude that Asx functions normally to repress Antp and to restrict wg expression in specific regions of the developing disc. We also show that, in the absence of Asx's function, Antp expression does not lead to efficient repression of the antennal-determining gene hth, suggesting that Asx is also required for the repression of hth by Antp.

Complex genetic interactions govern the temporal effects of Antennapedia on antenna-to-leg transformations in Drosophila melanogaster

The putative regulatory relationships between Antennapedia (Antp), spalt major (salm) and homothorax (hth) are tested with regard to the sensitive period of antenna-to-leg transformations. Although Antp expression repressed hth as predicted, contrary to expectations, hth did not show increased repression at higher Antp doses, whereas salm, a gene downstream of hth, did show such a dose response. Loss of hth allowed antenna-to-leg transformations but the relative timing of proximal-distal transformations was reversed, relative to transformations induced by ectopic Antp. Finally, overexpression of Hth was only partially able to rescue transformations induced by ectopic Antp. These results indicate that there may be additional molecules involved in antenna/leg identity and that spatial, temporal and dosage relationships are more subtle than suspected and must be part of a robust understanding of molecular network behaviour involved in determining appendage identity in Drosophila melanogaster.

The aryl hydrocarbon receptor sans xenobiotics: endogenous function in genetic model systems

For more than 30 years, the aryl hydrocarbon receptor [Ah receptor (AHR)] has been extensively scrutinized as the cellular receptor for numerous environmental contaminants, including polychlorinated dioxins, dibenzofurans, and biphenyls. Recent evidence argues that this description is incomplete and perhaps myopic. Ah receptor orthologs have been demonstrated to mediate diverse endogenous functions in our close vertebrate relatives as well as our distant invertebrate ancestors. Moreover, these endogenous functions suggest that xenobiotic toxicity may be best understood in the context of intrinsic AHR physiology. In this literature review, we survey the emerging picture of endogenous AHR biology from work in the vertebrate and invertebrate model systems Mus musculus, Caenorhabditis elegans, and Drosophila melanogaster.

distal antenna and distal antenna-related function in the retinal determination network during eye development in Drosophila

Drosophila eye specification occurs through the activity of the retinal determination (RD) network, which includes the Eyeless (Ey), Eyes absent (Eya), Sine oculis (So) and Dachshund (Dac) transcription factors. Based on their abilities to transform antennal precursors towards an eye fate, the distal antenna (dan) and distal antenna-related (danr) genes encode two new RD factors. Dan and Danr are probable transcription factors localized in nuclei of eye precursors and differentiating eye tissue. Loss-of-function single and double dan/danr mutants have small, rough eyes, indicating a requirement for wild-type eye development. In addition, dan and danr participate in the transcriptional hierarchy that controls expression of RD genes, and Dan and Danr interact physically and genetically with Ey and Dac. Eye specification culminates in differentiation of ommatidia through the activities of the proneural gene atonal (ato) in the founding R8 photoreceptor and Egfr signaling in additional photoreceptors. Danr expression overlaps with Ato during R8 specification, and Dan and Danr regulate Ato expression and are required for normal R8 induction and differentiation. These data demonstrate a role for Dan and Danr in eye development and provide a link between eye specification and differentiation.

The aryl hydrocarbon receptor, more than a xenobiotic-interacting protein

The aryl hydrocarbon (dioxin) receptor (AhR) has been studied for several decades largely because of its critical role in xenobiotic-induced toxicity and carcinogenesis. Albeit this is a major issue in basic and clinical research, an increasing number of investigators are turning their efforts to try to understand the physiology of the AhR under normal cellular conditions. This is an exciting area that covers cell proliferation and differentiation, endogenous mechanisms of activation, gene regulation, tumor development and cell motility and migration, among others. In this review, we will attempt to summarize the studies supporting the implication of the AhR in those endogenous cellular processes.

Misexpression screen in Drosophila melanogaster aiming to reveal novel factors involved in formation of body parts

To identify novel factors that lead a fly imaginal disc to adopt its developmental fate, we carried out a modular dominant misexpression screen in imaginal discs. We have identified two factors that appear to change the fate of the respective body structure and appear to lead to the transformation of a body part. In one mutant line, notum tissue, normally derived from wing imaginal tissue, formed close to the site of the sternopleural bristles, which are leg disc derivatives. In the other line, the arista is transformed into a tubular structure, resembling an abnormal leg. We found that ectopic expression of abrupt was responsible for this potential transformation of the arista.

Regulation of the Drosophila distal antennal determinant spineless

The transformation of antenna to leg is a classical model for understanding segmental fate decisions in Drosophila. The spineless (ss) gene encodes a bHLH-PAS transcription factor that plays a key role in specifying the identity of distal antennal segments. In this report, we identify the antennal disc enhancer of ss and then use enhancer-lacZ reporters to work out how ss antennal expression is regulated. The antennal determinants Distal-less (Dll) and homothorax (hth) are key activators of the antennal enhancer. Dll is required continuously and, when present at elevated levels, can activate the enhancer in regions devoid of hth expression. In contrast, homothorax (hth) is required only transiently both for activation of the enhancer and for specification of the aristal portion of the antenna. The antennal enhancer is repressed by cut, which determines its proximal limit of expression, and by ectopic Antennapedia (Antp). Repression by Antp is not mediated by hth, suggesting that ss may be a direct target of Antp. Finally, we show that ss+ is not a purely passive target of its regulators: ss+ partially represses hth in the third antennal segment and lies upstream of Dll in the development of the maxillary palp primordia.

The bHLH-PAS protein Spineless is necessary for the diversification of dendrite morphology of Drosophila dendritic arborization neurons

Dendrites exhibit a wide range of morphological diversity, and their arborization patterns are critical determinants of proper neural connectivity. How different neurons acquire their distinct dendritic branching patterns during development is not well understood. Here we report that Spineless (Ss), the Drosophila homolog of the mammalian aryl hydrocarbon (dioxin) receptor (Ahr), regulates dendrite diversity in the dendritic arborization (da) sensory neurons. In loss-of-function ss mutants, class I and II da neurons, which are normally characterized by their simple dendrite morphologies, elaborate more complex arbors, whereas the normally complex class III and IV da neurons develop simpler dendritic arbors. Consequently, different classes of da neurons elaborate dendrites with similar morphologies. In its control of dendritic diversity among da neurons, ss likely acts independently of its known cofactor tango and through a regulatory program distinct from those involving cut and abrupt. These findings suggest that one evolutionarily conserved role for Ahr in neuronal development concerns the diversification of dendrite morphology.

The specification of a highly derived arthropod appendage, the Drosophila labial palps, requires the joint action of selectors and signaling pathways

The remarkable diversity of form in arthropods reflects flexible genetic programs deploying many conserved genes. In the insect model Drosophila melanogaster, diversity of form can be observed between serially homologous appendages or when a single appendage is transformed by homeotic mutations, such as the adult labial mouthparts that can present alternative antennal, prothoracic, or maxillary identities. We have examined the roles of the Hox selector genes proboscipedia (pb) and Sex combs reduced (Scr), and the antennal selectors homothorax (hth) and spineless (ss) in labial specification, by tissue-directed mitotic recombination. Whereas loss of pb function transforms labium to prothoracic leg, loss of Scr, hth, or ss functions results in little or no change in labial specification. Results of analysis of single and multiple mutant combinations support a genetic hierarchy in which the homeotic pb gene possesses a primary role. It is surprising to note that while loss of ss activity alone had no detected effect, all mutant combinations lacking both pb and ss yielded the most severe phenotype observed: stunted, apparently tripartite legs that may correspond to a default state. The roles of the four selector genes are functionally linked to a cell nonautonomous mechanism involving the coupled activities of the decapentaplegic (dpp)/TGF-beta and wingless (wg)/Wnt signaling pathways. Accordingly, several mutant combinations impaired in dpp signaling were seen to reorient labial-to-leg transformations toward antennal aristae. A crucial aspect of selector function in development and evolution may be in regulating diffusible signals, including those emitted by dpp and wg.

Insect appendages and comparative ontogenetics

It is arguable that the evolutionary and ecological success of insects is due in large part to the versatility of their articulated appendages. Recent advances in our understanding of appendage development in Drosophila melanogaster, as well as functional and expression studies in other insect species have begun to frame the general themes of appendage development in the insects. Here, we review current studies that provide for a comparison of limb developmental mechanisms acting at five levels: (1) the specification of ventral appendage primordia; (2) specification of the limb axes; (3) regulation and interactions of genes expressed in specific domains of the proximal-distal axis, such as Distal-less; (4) the specification of appendage identity; and (5) genetic regulation of appendage allometry.